1. Field of the Invention
The present invention relates to an optical pickup used in an apparatus which records and reproduces utilizing an optical memory such as a compact disc player or a video disc player, and, more particularly, to an optical pickup using holographic optical element in an optical system in order to reduce size and weight.
2. Description of the Prior Art
FIG. 8 is a view showing a structure of a conventional optical pickup using a diffraction grating. In FIG. 8, light outputted from a semiconductor laser (laser diode) 34 is inputted to a first diffraction grating (referred to as a grating hereinafter) 35 and then separated to zero-order diffracted light (referred to as a main beam hereinafter) for reading an information signal and a pair of .+-.1-order diffracted light (referred to as a sub-beam hereinafter) for tracking which separates in a direction almost vertical to a paper surface. These three beams pass through a second diffraction grating (referred to as a hologram hereinafter) 36, a collimator lens 37 and an objective lens 38 and then converge at a recording carrier (disc) 39. Then, the reflected light from the recording carrier 39 passes through the objective lens 38 and the collimator lens 37 and then diffracted by the hologram 36 with the primary diffracted light being introduced to a photodetector 31.
FIGS. 9 and 10 show a grating pattern of the hologram 36 viewed from the recording carrier 39 and a pattern at a light receiving part of the photodetector 31, respectively. The hologram 36 comprises two regions 36a and 36b and the receiving part of the receiving element 31 comprises five regions 31a to 31e. The main beam diffracted in the region 36a of the hologram converges on a line between the light receiving parts 31a and 31b and the main beam diffracted in the region 36b converges in the light receiving part 31c. Then, two sub-beams converge in the light receiving parts 31d and 31e, respectively. Thus received light is converted to a current signal corresponding to a light intensity by the photodetector 31.
A current corresponding to each quantity of received light is generated from the light receiving parts 31a to 31e in the photodetector 31. The current is introduced into a current-voltage converting circuit positioned outside the optical pickup and converted to a voltage signal. Thereafter, a focus error signal (FES), a radial error signal (RES) and an information signal (RF) are generated by a signal operation circuit connected to the output of the current-voltage converting circuit. At this time, when signals outputted from the light receiving part 31a, 31b, 31c, 31d and 31e are designated by S.sub.31a, S.sub.31b, S.sub.31c, S.sub.31d and S.sub.31e, FES=S.sub.31a -S.sub.31b, RES=S.sub.31d -S.sub.31e and RF=S.sub.31a +S.sub.31b +S.sub.31c.
A structure of such optical pickup using holographic optical element is disclosed in "Optical Pickup" in Japanese Patent Laid Open No. 151022/1989.
However, optical utilization factor of the above optical pickup is lower, while the above optical pickup is smaller and its weight is less as compared with an optical pickup using a beam splitter of a half mirror instead of a diffraction grating. More specifically, when the half mirror is used, a quantity of light inputted to the photodetector becomes 25% when the light passes through the half mirror both ways. However, according to the pickup using the diffraction grating, a quantity of light is 14% at most when the light passes through the diffraction grating both ways. Thus, when the quantity of light becomes about half, an S/N ratio is deteriorated. The S/N ratio is likely to be deteriorated in process of transmitting a signal from the photodetector 31 to the current-voltage converting circuit by a small current.
In addition, when the signal processing circuit used together with the optical pickup is integrated corresponding to a signal level of the half mirror type optical pickup, the optical pickup using the holographic optical element whose signal level is about 1/2 can not be connected. Therefore, when the optical pickup using the holographic optical element is used together with the signal processing circuit, an amplifying circuit for adjusting a gain has to be provided between the optical pickup and the signal processing circuit.